When you immerse an empty plastic bottle in a bucket of water, it comes above the surface of water. Why does this happen? How can it remain immersed in water and why?
When we immerse an empty plastic bottle in a bucket of water, the upward force (upthrust or buoyant force) exerted by water on the bottle is greater than its own weight, therefore it comes above the surface of water. To keep the bottle completely immersed, an external force which is equal to the difference between the upward force and the weight of the bottle, must be applied on the bottle in downward direction. This is because the upthrust on the bottle due to water must be balanced.